The antenna of the invention may be used for transmission and reception of signals. Whilst the invention will be described with reference to receiving signals, this is by way of example only. In addition, the invention will be described with reference to linear polarisation by way of example. Polarisation such as circular, elliptical, horizontal and vertical polarisation are not excluded.
The antenna of the invention is particularly suited for satellite signal reception and the invention will be described by way of example with reference to this application.
Satellite broadcasting systems employ a satellite station at which signals are received, down converted to a frequency typically about 11 or 12 GHz and boosted by high power amplifiers before re-transmission back to earth. By the time the downlink signals reach the earth, they are extremely weak and specialist equipment is necessary to produce acceptable received signals.
Direct broadcasting by satellite (DBS) is now used for domestic purposes and adequate signals may be received in some areas by using a 65 cm dish antenna. For effective reception, the antenna should be small, easy to construct and consistently receive signals for most of the time. The antenna should have high gain, high directivity, good efficiency and a narrow bandwidth.
Flat-plate array antennas have been proposed for receiving DBS transmissions. For high power DBS applications, flat plate antennas offer an attractive alternative to conventional reflectors in terms of aesthetics, ease of installation and maintenance.
In conventional linearly polarised flat RLSA antennas, reflections occur in radiating slots provided in a flat plate. Such RLSA antennas were proposed and used in Japan. Various designs of both a circular and linearly polarised were suggested. One known RLSA circular polarised antenna had a radial waveguide with radiating slots arranged spirally which produced a circular polarised broadside beam. In order to increase efficiency, the upper part of the waveguide was filled with a dielectric material to suppress grating lobes in the array. Reception or transmission was at the centre of the lower waveguide via a coaxial cable.
A single layer RLSA antenna was also proposed. Its operation was similar to the double layer structure referred to above and had slots arranged in a spiral pattern in a plate provided on the antenna.
An RLSA antenna having slots arranged in concentric paths on the plate was also previously suggested.
Linearly polarised RLSA antennas are also known and produce a linearly polarised broadside beam. The configuration of such antennas was similar to circular polarised RLSA antennas except that the slots were annularly arranged. Such linearly polarised antennas exhibited poor return loss characteristics. The main reason for this was that slot reflections were added in phase since the slots were arranged for linear polarisation.
One technique for improving the return loss of linearly polarised RLSA antennas employed a beam tilting technique. An improvement in the return loss of 10 dB was reported for a tilt angle of 10.degree.. However, reflections from slots were not greatly suppressed.
A technique known as reflection cancelling has been suggested to suppress reflections caused by slots in the antenna. This method involved using additional slots spaced .lambda..sub.g /4 from the radiating slots for cancellation of reflections, where .lambda..sub.g is the guide wavelength. It is possible to provide an antenna with reflection cancelling slots and non-uniform slots for providing optimum performance but this is difficult, particularly since the respective slots should not overlap. In addition, slot coupling control is not fully realised by using nonuniform slots not designed to optimum length to avoid overlapping.